It's simple really. Make sure the racing is put on for fun and in a competitive but not moniacal fassion. Make the party at least as good as the racing and make sure the event is kept clean and with a feeling of "family" and you'll find Dad's and Mom's a lot more willing to make it a family sport again.

Have a crusiers division where you only have to complete 60% of the races and gag trophies are given. (but good ones)